Error HDL : Sel has no source pin

Error HDL : Sel has no source pin

,here is my HDL code for a Mux ;

CHIP Mux {
IN a, b, sel;
OUT out;

PARTS:

Not (in = Sel , out = notSel);
And(a = a , b = notSel , out = v1);
And (a = Sel , b = b , out = v2);
Or (a = v1 , b = v2 , out = out);
// Put your code here:
}
when i try to open the file in the harware simulatur , i get the error : Sel has no source pin ,.
I have a Sel pin defined in the Input part in my code , i don't understand the origine of this error .
I need a help